With all the talk about fact-based films and how accurate they should or shouldn’t be, it’s worth noting that some stories are best brought to screen as simply and purely as possible.

This is especially true with a film like “Lone Survivor,” Peter Berg’s expertly rendered account of a disastrous 2005 military operation in Afghanistan. War is messy, and politics are messy. But Berg has wisely chosen to focus pretty squarely on the action, and to present it as straightforwardly as possible.

And he’s executed that approach with admirable skill, down to using autopsy reports to get the number of wounds a soldier suffered exactly right. “Lone Survivor” doesn’t have nearly the sweep of a major war film like Spielberg’s “Saving Private Ryan.” But the action scenes – basically, one protracted, harrowing firefight – feel as realistic as any we’ve seen on the screen for some time.

That firefight, for those unfamiliar with the story (Berg also penned the screenplay, based on the memoir by former Navy SEAL Marcus Luttrell), took place June 28, 2005, in the craggy mountains of Afghanistan’s Kunar province. As part of Operation Red Wings, Luttrell and three fellow SEALS were positioned on a hillside, tracking a Taliban commander in the village below, when they suddenly encountered a few local shepherds. Their agonized decision on what to do with those shepherds, one of them a teenager, led to a string of events that ultimately resulted in 19 American deaths.

Of course, the title, “Lone Survivor,” and the fact Luttrell is played by the movie’s star (Mark Wahlberg, in a strong and moving performance) tells you much of what’s going to happen from the get-go. But that doesn’t hurt the film’s immediacy and power. In fact, you may have a hard time sitting still.

Berg opens with footage of real Navy SEAL training and the extremes it reaches – some might call it unnecessary and overly worshipful, but for people who don’t know a lot about the SEALS, it’s helpful and effective.

We’re also given a sense of the lighthearted camaraderie at the military base, in between operations, as the men joke about wives and girlfriends back home, or compete in foot races. One of the SEALS worries about how to afford a wedding present for his bride. The veterans engage in a little good-natured ribbing of a new arrival – involving some silly dancing.

But all lightness disappears suddenly, and for good. Soon, Luttrell is hunkered in the mountains with his comrades: Michael Murphy (Taylor Kitsch), Danny Dietz (Emile Hirsch), and Matt “Axe” Axelson (Ben Foster). All seems to be going well until the moment they encounter the villagers; the ensuing debate is a painful one. Do they let them go and risk certain discovery? Or do they “terminate” the problem? The men also touch on a heavier question: what connection, in a deeper sense, do these shepherds have with the enemy?

But a decision comes, and then the battle, with the men literally falling down the mountainside, smashing repeatedly into rocks, their bodies gashed and broken. Several of them fight while shot and gravely wounded. One virtually sacrifices himself to call for help. A rescue effort goes catastrophically badly.

And then comes the amazing end to the story: How, and with whose help, Luttrell manages to survive to tell his tale. Although it’s a matter of record, we’ll keep the suspense alive here.

At the end, we see photos of the actual casualties of Operation Red Wings. It does not seem gratuitous, and no further explanation or exposition is given, or needed. Again, the best thing about Berg’s work here is its simplicity.